Molecular cloning of the five extant monogenic forms of rodent obesity identified critical molecules in the pathways regulating energy homeostasis in animals and human (1). The agouti (a) gene encodes a 131 amino acid peptide, agouti signaling protein (ASP) that is normally secreted only in the follicular cells of the dermal papilla of the skin (2-4). Mutations of agouti (e.g. Ay and Avy) that cause ectopic overexpression of ASP in the hypothalamus and skin, result in a pleiotropic syndrome that includes increased lean and adipose tissue, yellow pelage (coat), hyperinsulinemia, hyperphagia, and hyperglycemia (3-5). ASP antagonizes the binding of alpha-melanocortin stimulating hormone (α-MSH) to the melanocortin 1 receptor (MC1R) causing melanogenesis to switch from the production of black/brown pigment (eumelanin) to a yellow/red pigment (pheomelanin) (FIG. 1) (2). In the hypothalamus, agouti related protein (AgRP) is the natural antagonist of MC3R and MC4R. AgRP also acts as an inverse agonist at MC4R in both human and mouse (6, 7). In Ay mice, ectopically produced ASP competes with α-MSH for binding to MC3R and MC4R, resulting in hyperphagia, increased body fat, and disordered insulin homeostasis (8).
Two other mutations, mahogany (Atrnmg, Chr 2) and mahoganoid (md, Chr 16), affect coat color and body weight. Both mutations are unique in their ability to suppress the obesity and darken the coat (umbrous effect) caused by mutations resulting in the ectopic overexpression of ASP. There are three reported alleles of mahogany: Atrnmg, Atrnmg-L, and Atrnmg-3J that are coisogenic on mouse strains LDJ/Le, C3H/HeJ, and C3HeB/FeJ, respectively (9, 10). Atrnmg and Atrnmg-L are each the result of ˜5 kb retrovirus insertions in introns 26 and 27, respectively, that disrupt the splicing of Atrn. Atrnmg-3J has a 5-bp deletion at nucleotide 2,809 introducing a stop codon that results in a severely truncated protein. Attractin (Atrn) encodes a single-pass transmembrane ˜210 kDa protein containing 3 epidermal growth factor (EGF) domains, two laminin-like EGF repeats, a CUB domain, two plexin-like repeats, a C-type lectin and seven consecutive Kelch repeats (9, 10). The predicted structure of ATRN suggests that it functions as a receptor or receptor-like protein. The Atrnmg mutation does not suppress the obese phenotype of Mc4r null mice, or that of several monogenic obese models (Leprdb, Lepob, tub, Cpefat) (9, 10), but does suppress diet-induced obesity (10). Homozygosity for LDJ/Le Atrnmg backcrossed for 6-8 generations onto a C57BL/6J background suppresses Ay-induced weight gain by increasing basal metabolic rate (11). In animals doubly mutant for mg and Ay, food intake is not reduced relative to controls (+/+ Ay/a and mg/mg a/a), but body weight is reduced due to higher metabolic rate (11). Mahogany mice are not hyperphagic on a C3H/HeJ background (12). Homozygous Atrnmg animals develop abnormal myelination and vacuolization throughout the brain and spinal cord in association with severe tremors and flaccid paresis (13). The tremors may account for the increased metabolic rate. The neuropathological changes characteristic of Atrnmg animals, and the specificity of binding of ASP but not AgRP to ATRN (14), make it unlikely that ATRN plays a specific role in hypothalamic control of energy homeostasis aside from its effects on muscle motor activity.
Md has effects on coat color and obesity in Ay mice that are analogous to Atrnmg. The md mutation originally arose spontaneously in the C3H/HeJ strain at Jackson Laboratory in the early 1960s. Subsequently, four additional spontaneous mutations (md2J, md4J, md5J, and md6J) have been documented at this locus Like Atrnmg, mahoganoid darkens the back, ears, and tail of nonalbino mice (15, 16) This darkening effect is described as an “umbrous” coat. Md suppresses Ay-induced yellow pigmentation and Ay-induced obesity in a gene dose-dependent manner (17) Similar to Atrnmg on the C57BL/6J background, homozygosity for md on the C3H/HeJ background causes hyperphagia (11). The ability of md to induce hyperphagia suggests that the md gene product has effects on energy homeostasis distinct from epistatic effects in the context of overexpression of ASP (Ay) Genetic studies have positioned md, functionally, at the same level or upstream of MC1R and downstream of ASP based on findings that the Mc1re mutation (extension, resulting in a yellow coat) suppressed the coat color effect of md, and that md suppressed both the yellow and obese phenotypes of Ay mice (17) (FIG. 1). Linkage analysis positioned md on chromosome 16, about 2 cM from the centromere (18, 19) Identification of md and its function could provide additional insight into the control of melanocortin signaling.